1. Field of the Invention
The present invention relates to a single mode optical fiber and more particularly to a single mode optical fiber for use in optical communications that is not susceptible to loss by bending when said fiber is formed into a cable and can cause a chromatic dispersion, which is a cause of deterioration in the transmission bandwidth, to be zero at a wavelength of approximately 1.5.mu.m at which optical fiber loss is minimized.
2. DESCRIPTION OF THE PRIOR ART
A condition of light propagating in an optical fiber is determined by a normalized frequency V. When the wavelength being used is .lambda., V is given by the following equation. ##EQU1## In the above equation, n.sub.m is a core refractive index and a is a core radius. .DELTA. is a value of a relative refractive index difference defined as ##EQU2## when a refractive index of a cladding is n.sub.2. It is known that in a step-index optical fiber, the optical fiber is in single mode when V&lt;2.4 and the transmission bandwidth of a single-mode optical fiber is limited by chromatic dispersion. The chromatic dispersion is given by the sum of the material dispersion dependent on the fiber material and the waveguide dispersion caused by the refractive index profile of the fiber.
The material dispersion of silica optical fiber is positive in a longer wavelength region of a wavelength over 1.3.mu.m. On the other hand, the waveguide dispersion is negative in a so-called single mode region in the case of a step-type fiber. Consequently, it is clear that at a wavelength over 1.3.mu.m the chromatic dispersion given by the sum of these values can be made to be zero. On the other hand, the chromatic dispersion of a usual single mode optical fiber (step-type) designed for 1.3.mu.m band (.DELTA.=0.003, 2a=10.mu.m) is a large value of 16-20ps/km/nm in the 1.5.mu.m wavelength region, so that such fiber is not suited to optical communication requiring ultra-wide bandwidth. Therefore, in order to make the dispersion zero in the 1.5.mu.m wavelength region (1.51-1.59.mu.m), it is sufficient that .DELTA. be larger than 0.004 in the vicinity of V.perspectiveto.1 for step-type optical fiber (alpha index profile type). In this case, the radius of the core is small so that the arrangement is likely to have a larger bending loss.
The following approximation can be made for a splice loss .alpha..sub.s with respect to an axial displacement d of a fiber. EQU .alpha..sub.s =4.3 (d/W).sup.2 [dB]
In this equation, W indicates a mode field radius. Consequently, when the axial displacement d is constant, the splice loss .alpha.s becomes smaller as the mode field diameter 2W increases. Furthermore, as the mode field diameter 2W becomes smaller, a power is better confined inside the core, so that the bending loss decreases. However, when the mode field diameter 2W is large, the splice loss .alpha..sub.s decreases, but the bending loss increases. Consequently, the relationship between the bending loss and the splice loss is traded off against the size of the mode field diameter 2W. For this reason, in an alpha index profile type 1.5.mu.m zero dispersion fiber, there is a disadvantage that the mode field diameter cannot be made large.
FIG. 1 illustrates the relationship between a mode field diameter 2W of a conventional alpha index type 1.5.mu.m band zero dispersion fiber and an allowable bending radius R*. The allowable bending radius R* is defined as the bending radius for the case in which a bending loss of 0.01dB/km occurs when an optical fiber is bent uniformly. Furthermore, the mode field diameter 2W is a parameter expressing an expansion of the field of the lowest mode propagating through the optical fiber. In a conventional 1.3.mu.m band zero dispersion fiber, the allowable bending radius R* at 1.3.mu.m is 4cm and in this case, it has been confirmed that there is no increase in loss when the fiber is formed into a cable. That is, R*=4cm is the standard of the allowable bending radius when optical fiber is formed into a core wire or a cable. As seen in FIG. 1, in an alpha-power index profile 1.5.mu.m band zero dispersion fiber, when the mode field diameter exceeds 8.mu.m, the allowable bending radius is larger than that for a conventional 1.3.mu.m band zero dispersion optical fiber at 1.3.mu.m, so that the arrangement is likely to increase the loss when the fiber is formed into a coated fiber or a cable.
To overcome this disadvantage, Japanese patent application Laying-open No. 53-97849 entitled "Single Mode Optical Fiber" laid open on Aug. 26, 1978 discloses an arrangement having improved bending loss characteristics in which an expansion of the field is made smaller than that of a conventional step-type index single mode optical fiber by making the refractive index of the center portion of the core larger than that of the remaining or peripheral portion of the core in a conventional step-type refractive index profile.
In this disclosure, however, there is a disadvantage that it is not possible to obtain zero dispersion and good bending characteristics as well as a reduced splice loss in the 1.5.mu.m wavelength region.
Furthermore, there is a disadvantage of poor manufacturability because of a large variation in the dispersion value with respect to a variation in core diameter in the alpha index profile type 1.5.mu.m zero dispersion fiber.
In order to solve the above disadvantages, European patent application Laying-open No. 0127408 entitled "Optical Waveguide Fiber" laid open on May 12, 1984 has proposed a segment-core type zero dispersion fiber having a core composed of at least two concentric portions surrounding the center portion of the core and including one or more regions which are disposed between the two concentric portions and in which refractive indices are lower than the concentric portions. However, this fiber has a complicated refractive index profile, so that the control of the refractive index profile in the direction of the radius of the optical fiber is complicated in the fabrication process of the fiber. This means that it is difficult to control the refractive index profile.